An antimicrobial agent in household soap accumulates in the sludge produced by wastewater treatment plants, environmental engineers report. Farmers apply the sludge on fields. High doses of this compound, the antiseptic triclocarban, or TCC, impair reproduction in rats and may cause cancer in humans, but it's not clear whether the concentrations in fields--and presumably in food--are dangerous.

Researchers had previously shown that TCC passes through wastewater treatment plants into streams, where levels are safe. Researchers had struggled to accurately measure TCC in sewage sludge, however, because they lacked adequate detection techniques. Now, a technique called tandem mass spectrometry has cleared up the problem.

Environmental engineer Rolf Halden and co-authors at Johns Hopkins University Bloomberg School of Public Health in Baltimore, Maryland, gathered samples from an urban sewage treatment facility in the eastern United States, measuring the amount of TCC entering and exiting the treatment plant. As in an earlier study, the results showed that the facility discharged 3% of incoming TCC into surface water at a level of a few parts per billion. Nearly all of this residual TCC retained its potency.

The concentration of TCC in sludge soared to 10 to 70 parts per million--roughly equal to the concentration in hand soap--the researchers report in the 1 June issue of Environmental Science and Technology. Ninety-five percent of the sludge is applied to farm fields as soil conditioner and crop fertilizer, leading to the release from a single facility of nearly 1 metric ton of TCC into the environment each year, the researchers calculate. The United States has 18,000 such facilities. "When we try to do the right thing by recycling nutrients contained in biosolids, we end up spreading a known reproductive toxicant on the soil where we grow our food," Halden says.

"Finding these chemicals is like finding a needle in the haystack," says Shane Snyder, project manager at Southern Nevada Water Authority in Las Vegas, Nevada. Tandem mass spectrometry, he adds, is "absolutely the best way of monitoring this contaminant. It's the way of the future." Although it's not on the Environmental Protection Agency's list of persistent organic pollutants, Snyder says, the data from this study could help determine whether the use of TCC should be restricted.